We were forced from Alltel to AT&T this spring, and got the freebie phone that was offered, which is, frankly, a rather crappy phone. But, this is the first phone I've ever had with a SIM card. Does that card mean I can get any phone with a SIM-card slot, insert the SIM, and - viola - it's suddenly my phone?

I like the idea that I could just pick something up off Craigslist or eBay and switch my phone without hoops to jump through...but my guess is that there are always hoops. Let me know what to watch out for, what is and isn't possible, etc. Also, I understand that SIM cards come in two sizes: mine is the bigger one.

Yes, you can put this SIM in another phone with a SIM slot and it will now have your number.

Some phones are locked to a specific network and will not work with a SIM from another network. These phones can usually be unlocked cheaply by asking the original network (possibly), by going to a back street phone shop, or by paying for an unlock code on the internet.

You can usually save your contacts to the SIM card (a limited number of contacts with limited length fields) and then import them again on another phone.

The phone should be able to make emergency calls (911 or 112 or whatever is your emergency number) even without a SIM in it.posted by emilyw at 1:59 PM on July 22, 2011

You have to be a little bit careful to get a GSM type phone that has 850/1900 mhz frequencies (both are used in the AT&T network, and not all GSM phones have these specific frequency bands), and that the phone you buy is "unlocked" or at least not locked to another carrier in its internal software. And obviously, the phone you buy has to be compatible with the type of SIM card you have, too. See this AT&T Support forum thread for details.posted by paulsc at 2:01 PM on July 22, 2011

What emilyw and paulsc said -- eBay sellers will usually state the operating frequencies and whether a GSM phone is vendor-locked; Craigslist is more of a lottery, though since there are only two big GSM operators in the US, you can often be guided by the vendor branding. T-Mobile and AT&T also use different 3G frequencies, so if you want an unlocked smartphone (not cheap) you'll need to check that too.

microSIMs are only a factor on high-end devices, and you can actually cut down your SIM to the microSIM size.posted by holgate at 2:06 PM on July 22, 2011

The only other thing I can add is that the carrier may require certain data plans for some phones. In my experience, I had a phone that required a more expensive data plan, and I received a text telling me I needed to update my data plan about six months later.

Disclaimer: I work for a major telco, but not in the mobile division. YMMV, IANAWhatver...posted by Mad_Carew at 4:55 PM on July 22, 2011

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